Static compensators for reactive power (Static Var Compensators—SVC) are conventionally used in high-voltage electric transmission networks as a tool for rapid control of the voltage, the compensators being connected to the network in particular in nodes where the network is weak.
In an article in a journal entitled: Static Var Compensator Models for Power Flow and Dynamic Performance Simulation. IEEE Special Stability Controls Working Group. IEE Transactions on Power Systems, Vol. 9, No. 1, February 1994, pages 229–240, various kinds of compensators for the purpose as well as their control systems are described.
As examples of such compensators, the above-mentioned article mentions, among other things, thyristor-controlled reactors (TCR), but recently compensators based on voltage-source converters (VSC) have also been used.
The thyristor-controlled reactor enables a controllable consumption of reactive power and comprises an inductive element, a reactor, in series connection with a controllable semiconductor valve. The controllable semiconductor valve comprises two controllable semiconductors, usually thyristors, in antiparallel connection. By phase-angle control of the semiconductors, that is, by controlling their firing angle relative to the phase position for the voltage of the ac network, the susceptance of the reactor, and hence its consumption of reactive power, may be controlled
For a general description of thyristor-controlled reactors, reference is made to {dot over (A)}ke Ekström: High Power Electronics HVDC and SVC, Stockholm June 1990, in particular to pages 1-32 to 1-33 and 10-8 to 10-12.
A thyristor-controlled reactor can only consume reactive power, and a compensator based on this technology therefore usually comprises devices for generation of reactive power. Such devices normally comprise one or more mutually parallel-connected filters, each one essentially comprising an inductive element in series connection with capacitive elements and tuned to one or more chosen multiples of the nominal frequency of the ac network, but in many cases capacitor banks, which may be connected in steps, are also included.
Components in the compensator which lie at a high-power level, such as, for example, a thyristor-controlled reactor and the above-mentioned devices for generating reactive power, are usually connected to a busbar at medium-voltage level, typically 15–30 kV, which busbar is connected via a transformer to the high-voltage, typically 130–800 kV, node in the transmission network.
The control system is based on sensing a voltage in the transmission network, usually at the node where the compensator is connected, and comprises a voltage controller for maintaining this voltage constant, generally with built-in statics in dependence on the current through the compensator, as shown in FIG. 2 in the above-mentioned article.
The voltage controller usually has an integrating characteristic and forms an output signal in dependence on the difference between a supplied reference value of the voltage at a point in the network and a similarly supplied actual value, sensed in the network, of this voltage. The output signal of the voltage controller is used as a reference value for the reactive power flow through the compensator. In those cases where the compensator comprises a thyristor-controlled reactor, the output signal from the voltage controller may easily be transformed into a reference value for the susceptance of the thyristor-controlled reactor.
According to the prior art, which will be described in greater detail with reference to the following description of an embodiment of the invention, the voltage in all of the three phases of the network is sensed, whereupon the sensed three-phase voltage is transformed into a two-phase system of coordinates rotating with the angular frequency of the network. In this system of coordinates, the sensed three-phase voltage is represented by a voltage vector which, under stationary conditions, is stationary in the rotating system of coordinates. The magnitude of this voltage vector here constitutes that actual value of the voltage which is supplied to the voltage controller. By a suitable synchronization of the rotating system of coordinates, the sensed voltage is reproduced as a vector which, under stationary conditions, only has a component directed along one axis of the system of coordinates, in the following called the q-axis or the transverse axis, whereas the component of the voltage along the axis orthogonal with the q-axis, in the following called the d-axis or the longitudinal axis, thus has the length zero.
However, it has proved that in cases where the rated power of the compensator corresponds to a considerable part of the short-circuit power of the network at the connection point, typically 25% or more, a control system as the one described above is not capable of controlling the voltage in a satisfactory manner. Normally, the lowest resonant frequency is greater than twice the system frequency, that is, greater than 100 Hz at a system frequency of 50 Hz, but under the conditions mentioned the resonant frequency, especially in the case of long lines and for certain circuit configurations, may approach the system frequency. This implies that, for stability reasons, the amplification of the voltage controller has to be reduced.
In particular, it has proved that, in the case of sudden voltage changes, such as for example switching operations for connecting long lines into the network, overvoltages may in certain case arise, with an ensuing risk of destroying apparatus connected to the network. After an initial reduction of the voltage at the connection point, the connection of a long idling line then results in an overvoltage which remains for a relatively long period of time.
A conventional method of overcoming these problems would be to introduce a proportional-gain characteristic in the voltage controller, which must then in general also be combined with stabilizing measures, such as the introduction of a differentiating characteristic in the controller. However, this has proved not to be a suitable solution, especially not in those cases where the line is to be connected into a transmission network which may exhibit different circuit configurations at different switching times, since in general it is then not possible to find controller settings which give a satisfactory result in all operating cases.